Air traffic management in general and the regulations concerning safety and therefore concerning separation from the relief and segregation between aircraft have for a very long time required the state organisations and airport authorities to publish take-off or landing procedures guaranteeing the safety of the flights leaving from or arriving at the airports.
These graphic or text procedures have for a long time been available only in paper form. The advent of flight management systems having brought with it the need to electronically manage all the take-off or landing procedures published by the states.
Currently, the textual and graphical procedures are supplied by the member states of the International Civil Aviation organization to the suppliers of navigation databases and are converted by the suppliers into series of legs. A leg is a flight plan portion defined by certain parameters (for example: position, altitude, heading/route rules). The coding rules for civil aviation are described in an international document published by the ARINC Committee (document ARINC 424). The current standard is issue 17 of this document.
FIG. 1 represents a diagram of a trajectory determination method according to the prior art. This method comprises: the design of flight procedures 102 from raw data 101 obtained from the states. This step is performed using dedicated design tools such as GeoTitan. These raw data enriched with procedures are coded 103 in the ARINC 424 standard, then integrated 104 in a flight management system. The flight management system uses these coded data as a basis for calculating 105 flight trajectories.
One of the most important principles in the production of navigation databases is that the data must not be corrupted, in other words the digitization method must not introduce degradations in the procedure.
The management of a trajectory from the published procedures therefore involves processing, in the flight management system, all the legs defined in ARINC424-17, or 20 legs and 3 holding patterns (race-track patterns), and above all, all sequencing combinations of these legs.
The legs currently defined are                so-called “fixed” legs, the termination of which is a waypoint that is published and fixed on the ground,        so-called “floating” legs, the termination of which is given by a variable condition (for example, altitude legs which terminate when the aeroplane has reached the altitude concerned), and        holding “procedure” legs (holding patterns, 3 types) and reversal procedure legs on approach (1 type).        There are eight “fixed” legs, eleven floating legs and four procedure legs.        
The table below gives the various legs:
LegNameMeaningIFInitial FixInitial point fixed on the groundCFCourse To a FixRejoin/follow a ground route to a fixed pointDFDirect to a FixDirectly (in a straight line) to rejoin a fixedpointTFTrack betweenGreat circle route between two fixed pointstwo FixesAFArc DME to a FixDefines an arc of circle around a DME beaconat a specified distance, with an aperture limitRFRadius to a FixDefines an arc of circle between 2 fixed points(the first point being the fixed point of thepreceding leg), on a centre of the fixed circleVIHeading toDefines a heading to be followed until the nextCIInterceptleg is interceptedCourse toDefines a route to be followed until the nextInterceptleg is interceptedVAHeading toDefines a heading to be followed to a givenCAAltitudealtitudeCourse toDefines a route to be followed to a givenAltitudealtitudeFAFix to AltitudeDefines a route to be followed, starting from afixed point, to a given altitudeVDHeading to DMEDefines a heading to be followed until aCDDistancespecified DME arc is interceptedCourse to DMEDefines a route to be followed until a specifiedDistanceDME arc is interceptedVRHeading to RadialDefines a heading to be followed until aCRCourse to Radialspecified radial is interceptedDefines a route to be followed until a specifiedradial is interceptedFCTrack from Fix toDefines a route to be followed starting from aFDDistancefix, over a specified distanceTrack from Fix toDefines a route to be followed starting from aDME Distancefix, until a DME arc is intercepted (DMEdistance specified)VMHeading toDefines a heading without termination (infiniteManualhalf-right)FMFix to ManualDefines a route, starting from a fix, withouttermination (infinite half-right)HARace-track circuit, with altitude exit conditionsHFRace-track circuit, with a single rotationHMManual race-track circuit, with no exitconditionPIFix to ManualSeparation procedure defined by a separationroute starting from a fix, followed by a half-turn, and interception of the initial separationroute for the return
In addition, the current standards limit the number of leg combinations by prohibiting certain leg sequences. Thus, at 529 possible leg combinations approximately only 360 are allowed. This very large number of procedures primarily has two negative impacts: trajectories are difficult to develop in the flight management systems because of this combination and the dispersion in terms of lateral position may be very significant with floating legs.